Caudate neurons of rat were tested by iontophoresis. Dopamine, apomorphine, cyclic AMP, ATP, 5'AMP, adenosine and 2-C1-adenosine all inhibit the spontaneous firing of caudate neurons. Results of studies using interactions of these agonist responses with phosphodiesterase inhibitiors, phenothiazines, methyl xanthines and prostaglandin E1 all suggest that these neurons possess two independent inhibitory receptors both of which may generate cyclic AMP postsynaptically. That a tonically firing inhibitory dopamine pathway projects to caudate from substantia nigra is suggested by studies with 6 OHDA, which removes this pathway.